Automatic Combustion Control – Control Strategy
ACC continually compares live process measurements to the constraint values obtained during heater testing. ACC slowly reduces excess air (closes the stack damper) until any measurement exceeds its constraint value. Then, ACC more rapidly increases excess air until the condition subsides. (The initial opening move is a step of 1 to 3% to overcome hysteresis (sticking) in the damper actuator system.) With no variable exceeding its constraint value, the damper is again slowly closed until the same, or a different constraint value is exceeded. The process then repeats.
ACC monitors the following process variables and compares them to the constraint values found during heater testing. If any values are exceeded as excess air is reduced, the damper is opened.
Flue gas O2 content. This gives a continuous relative efficiency indication. If O2 falls within a narrow range of O2 readings just above the constraint value (an “O2 Dead Band”), the closing of the damper stops, but the damper opens if any constraint condition appears. This avoids excessive damper and actuator movement while maintaining high efficiency operation. If O2 falls further (below its constraint value), the damper opens, admitting more air and raising the O2 level.
Flue gas CO content. CO is a sensitive measure of combustion completeness. An increasing CO level usually means that the heater is operating very close to maximum efficiency, but it could also mean that a burner is fouled or poorly adjusted, allowing unburned fuel into the heater. The CO constraint value is usually
set at 150 ppm.
Heater draft. Measured at the point of least negative pressure, usually at the bottom of the convection section. Pressure is never allowed to be positive. The draft constraint value is often set at -0.05 inch Water Column (WC).
Tube skin temperatures. A signal is sent to ACC to give more excess air when any skin temperature exceeds a preset value (chosen well below harmful limits).
Fuel flow. ACC produces a feed-forward signal to furnish more combustion air in proportion to fuel increases.
No attempt is made to do continuous CO or O2 control. The ACC “ramping” action overcomes delays and dead times associated with extractive flue gas analyzer systems. That is, closing the damper slowly enough to prevent fuel-rich excursions and opening it faster to provide slightly more air than minimally required.